US8379780B2 - Method for compensation of information losses in an OFDM-based multi-carrier communication signal caused by blanking out pulse-shaped interferences - Google Patents

Method for compensation of information losses in an OFDM-based multi-carrier communication signal caused by blanking out pulse-shaped interferences Download PDF

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US8379780B2
US8379780B2 US12/500,962 US50096209A US8379780B2 US 8379780 B2 US8379780 B2 US 8379780B2 US 50096209 A US50096209 A US 50096209A US 8379780 B2 US8379780 B2 US 8379780B2
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signal
pulse
subcarrier
ofdm
signals
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US20100008454A1 (en
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Michael Schnell
Sinja Brandes
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Deutsches Zentrum fuer Luft und Raumfahrt eV
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/0202—Channel estimation
    • H04L25/022—Channel estimation of frequency response
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
    • H04L25/03006—Arrangements for removing intersymbol interference
    • H04L25/03159—Arrangements for removing intersymbol interference operating in the frequency domain
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
    • H04L1/0045—Arrangements at the receiver end

Definitions

  • the present invention relates to a method for compensation of information losses, which have been caused by blanking out pulse-shaped interferences, in a communication signal received by a receiver.
  • the invention relates to the application of said compensation method in communication signals on the aviation sector, namely in systems based on “Orthogonal Frequency-Division Multiplexing” (OFDM).
  • OFDM Orthogonal Frequency-Division Multiplexing
  • the number of frequency bands available for aeronautical communication is limited so that the increasing demand for communication makes it necessary to search for new solutions.
  • One possible approach is seen in using that range of the frequency bands which serves, inter alia, for operation of ground-based systems provided for distance and/or position determination of aircraft, e.g. the so-called “Distance Measuring Equipment” (DME) or the “Tactical Air Navigation System” (TACAN) which is also employed for military purposes.
  • DME Distance Measuring Equipment
  • TACAN Tortactical Air Navigation System
  • JTIDS Joint Tactical Information Distribution System
  • MIMS Multifunctional Information Distribution System
  • the useful signal i.e. the communication signal
  • the useful signal is massively impaired by pulse-shaped interference signals (e.g. the DME signals) because the power level of the OFDM-based communication signal is considerably smaller than the power level of the DME signals.
  • pulse-shaped interference signals e.g. the DME signals
  • the above described problem can be formulated to the effect that, depending on the respective frequency band in which the OFDM-based system is operated, there will occur, at the receiver, pulse-shaped interferences from other systems which are operated in the same or the adjacent frequency band.
  • These interferences though being very short and thus impairing the OFDM signal only for a short time, mostly have a very high power and take a significant influence on the OFDM system.
  • the interference pulses and, respectively, the samples of the OFDM signal affected by the interference are easily detectable. If the detected power has a sample value above a specific threshold value, the respective sample value is set to zero or its amplitude is reduced to a threshold value.
  • the state of the art includes techniques provided to weaken pulse-interfering factors in communication or navigation systems. For this purpose, one will largely use the techniques of “pulse blanking” or “clipping” (amplitude limitation) or combinations of both techniques (see e.g. [2] and [3]).
  • the influence of the pulse-shaped interference is considerably reduced, thus effecting an improvement of the overall performance of the OFDM system.
  • the process will have an influence not only on the interference signal but also on the desired OFDM signal; this will entail losses in performance, causing a distinct reduction of the benefits obtained by pulse blanking or clipping.
  • the present invention has been designed to compensate for information losses in the frequency domain caused by blanking out pulse-shaped interferences in an OFDM-based multi-carrier communication signal received by a receiver.
  • the invention proposes a method for compensation of information losses caused by blanking out pulse-shaped interferences, in a communication signal received by a receiver, wherein, in said method,
  • pulse blanking When examining the thus changed OFDM-based signal by windowing, pulse blanking can be perceived as a rectangular window with notches. It is basically known that a signal resulting from a windowing of this type is then (when represented in the frequency domain) characterized in that the signal portions at each subcarrier frequency are obtained by an overlap of determinable signal portions respectively of all other subcarrier signals.
  • the signal portions are determinable and reconstructable because ICI in the OFDM-based signal can be determined from the shape of the subcarrier spectra derived from the pulse blanking window, estimates of the transmitted complex symbols on all subcarriers and estimates of the channel coefficients representing the influence of the transmission channel on all subcarriers.
  • ICI in the OFDM-based signal can be determined from the shape of the subcarrier spectra derived from the pulse blanking window, estimates of the transmitted complex symbols on all subcarriers and estimates of the channel coefficients representing the influence of the transmission channel on all subcarriers.
  • an OFDM symbol is transmitted by the complex symbols modulated on the individual subcarrier signals.
  • it is generally required to know the data symbol transmitted on each subcarrier as well as the respective channel influence on each subcarrier.
  • the transmitted sequence of data on the subcarrier signals—which sequence forms the data symbol— is to be iteratively determined by first equalizing, detecting and decoding the sequence of the received data, then coding and modulating said sequence again, for use as estimated values for the data symbol, onto the subcarrier signals, wherein these estimated values will be used during the compensation of the influence exerted by the blanking of the pulse-shaped interferences.
  • Symbols that are already known in the receiver such as null symbols in the guard band at the edges of the spectrum and pilot symbols employed for channel estimation are not estimated. Instead, the known values are directly used for the compensation of the influence of pulse blanking.
  • the influence of the transmission channel on each subcarrier is estimated by means of any conventional channel estimation algorithm.
  • the method of the invention is particularly suited for aeronautical communication if this communication is OFDM-based and is being overlapped e.g. by ground-based navigation systems for distance and/or position determination of an aircraft causing pulse-shaped interference signals at the receiver of the OFDM-based communication system.
  • it is useful to arrange the OFDM-based communication signals between the center frequencies of adjacent frequency channels of the system for distance and/or position determination of the aircraft.
  • FIG. 1 is a view of a pulse blanking operation performed by means of a rectangular window with notches
  • FIG. 2 is a graphical representation of the absolute value of the data symbols received on all subcarriers with and without pulse blanking (in a system with 64 subcarriers);
  • FIG. 3 is a diagram showing the bit error rate (BER) in relation to the signal-to-noise ratio (SNR) after pulse blanking with different numbers and lengths of portions set to zero;
  • FIG. 4 is a simplified block diagram of the OFDM receiver with compensation of pulse blanking
  • FIG. 5 is a diagram wherein the BER is plotted over SNR after compensation of the influence of pulse blanking assuming perfect channel knowledge
  • FIG. 6 is a diagram representing an example of an OFDM signal with 64 subcarriers, shown in the time domain, wherein the real part of the signals on the 6th to 8th subcarriers is represented in the time domain and without pulse blanking;
  • FIG. 8 is a diagram representing the spectra of the subcarriers 6 to 8 without pulse blanking.
  • the description will refer to a simple OFDM system with N subcarriers.
  • the desired OFDM signal and a pulse-shaped interference signal are present in mutual overlap.
  • B ⁇ N ⁇ 1 portions wherein the received time domain signal has been set to zero.
  • z 0 sample values have been set to zero
  • z 1 sample values in the last portion, Z B-1 sample values.
  • Pulse blanking can be represented as windowing with a rectangular window having notches at the corresponding positions.
  • the resulting window function is shown in FIG. 1 .
  • DFT discrete Fourier transform
  • FFT fast Fourier transform
  • FIG. 2 this is illustrated by way of a simple example.
  • the influence of the pulse blanking on the OFDM system can be explained by imagining the pulse blanking as a windowing by use of a rectangular window with notches at the respective positions.
  • the window function ( FIG. 1 ) can be represented as
  • w ⁇ [ k ] ⁇ 0 ⁇ k ⁇ B l , 0 , 1 B u , 0 ⁇ k ⁇ B l , 1 , ... ⁇ , B u , B - 1 ⁇ k ⁇ N - 1 0 otherwise ⁇ ( 1 )
  • B l,b and respectively B u,b denote the position of the last and respectively first sample value not set to zero, before and respectively after the b-th notch.
  • the subcarriers are not orthogonal to each other anymore, which will lead to inter-carrier interference (ICI).
  • ICI inter-carrier interference
  • the transmitted data sequence is iteratively estimated in the receiver in that the received sequence Y′[n] will first be detected and decoded and then will be coded and modulated onto complex symbols again.
  • These estimated values of the transmitted complex data symbols ⁇ circumflex over (d) ⁇ v are used in the compensation of the influence of pulse blanking according to equation (5).
  • Symbols that are already known in the receiver such as null symbols in the guard band at the edges of the spectrum and pilot symbols employed for channel estimation (see FIG.
  • FIG. 4 shows a simplified block diagram of an OFDM receiver in which the influence of pulse blanking will be compensated for.
  • the performance of the proposed compensation algorithm will now be demonstrated by way of a simple example.
  • the channel coefficients are assumed to be ‘1’, i.e. the channel is assumed to be perfectly known.
  • FIGS. 6 to 9 show an OFDM signal with and respectively without pulse blanking in the time domain and in the frequency domain.
  • FIG. 6 an OFDM signal without pulse blanking is shown in the time domain.
  • FIGS. 8 and 9 show equivalent representations of the above example according to FIGS. 6 and 7 in the frequency domain.
  • FIG. 8 for use as reference, the case without pulse blanking is shown in the frequency domain. It can be seen that the adjacent subcarriers because of their orthogonality have no influence on the observed 6th subcarrier.
  • the invention when used in OFDM-based systems, is distinguished by the compensation of the influence caused by pulse blanking.
  • the invention proposes a method which is suited to reverse the influence of pulse blanking in OFDM-based communication systems. Since this method makes use of specific characteristic properties of OFDM signals, such a compensation is possible only in OFDM-based multi-carrier communication systems and the method is applicable only in OFDM-based multi-carrier communication systems.
  • the invention can be used in cellular mobile and in aeronautical communication systems.
  • Current extensions of the cellular mobile communication standard (3GPP/LTE) and current and planned techniques for wired and wireless networks (WiFi, WiMAX and advanced versions thereof) involve the application of the OFDM technique.
  • the invention can render these systems robust against pulse-shaped interferences.
  • One proposed system is based on OFDM; since the L-band is subjected to heavy disturbances by pulse interferences, applying the invention would be particularly beneficial herein (cf. the problematics to be expected in future aeronautic communication as outlined in the introduction herein).

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Power Engineering (AREA)
  • Noise Elimination (AREA)
US12/500,962 2008-07-12 2009-07-10 Method for compensation of information losses in an OFDM-based multi-carrier communication signal caused by blanking out pulse-shaped interferences Active 2031-11-24 US8379780B2 (en)

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DE102008032913A DE102008032913A1 (de) 2008-07-12 2008-07-12 Verfahren zur Kompensation von durch Ausblendung pulsförmiger Störeinflüsse erzeugten Informationsverlusten in einem Kommunikationssignal
DE102008032913 2008-07-12
DE102008032913.4 2008-07-12

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DE102010007874B4 (de) 2010-02-13 2011-09-01 Deutsches Zentrum für Luft- und Raumfahrt e.V. Verfahren zur Reduktion von Informationsverlusten in einem OFDM-basierten Mehrträgersignal
GB201202571D0 (en) * 2012-02-15 2012-03-28 Pace Plc Improvements relating to adaptive impulsive noise migigation by using multi mode compressive sensing
CN104242964A (zh) * 2013-06-20 2014-12-24 中兴通讯股份有限公司 一种通信系统中抑制脉冲干扰的方法及装置
DE102015001106B4 (de) * 2015-02-01 2016-12-15 IAD Gesellschaft für Informatik, Automatisierung und Datenverarbeitung mbH Verfahren zur frequenz- und zeitselektiven Interferenzunterdrückung für ein Kommunikationssystem basierend auf OFDM und Empfänger hierfür
US10097255B2 (en) * 2015-07-01 2018-10-09 Qualcomm Incorporated Joint channel and phase noise estimation in control symbols of a millimeter wave link
CN115395982B (zh) * 2022-10-27 2023-01-24 北京智芯微电子科技有限公司 脉冲干扰的抑制方法、抑制系统、设备和芯片

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ATE523997T1 (de) 2011-09-15
EP2144414B1 (de) 2011-09-07
US20100008454A1 (en) 2010-01-14
EP2144414A1 (de) 2010-01-13
DE102008032913A1 (de) 2010-03-25

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